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Levothyroxine Sodium

Thyroid drugs & hormone Prescription
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Indications

Hypothyroidism Cancer

Synthox is used as replacement therapy for hypothyroidism of any cause. It should not be started for transient hypothyroidism seen during the recovery phase of subacute thyroiditis. It is also used to suppress Thyroid Stimulating Hormone (TSH) levels in the presence of goitres or nodules, and after radiological and/or surgical treatment of thyroid cancer, as well as to counter the goitrogenic effects of certain drugs such as lithium. It may additionally be used as a diagnostic aid in suppression tests.

⚠️ Always consult a registered physician before taking any medicine.

Pharmacology

This preparation contains synthetic Levothyroxine (also called Thyroxine or T4), which is identical to the natural hormone T4 produced in the Thyroid gland. About 30% of T4 is converted to the much more active Triiodothyronine (T3) in peripheral tissues. TBG (Thyroxine Binding Globulin) is the major carrier of T4. This binding protects T4 from metabolism and excretion, resulting in its long half-life in the circulation. Only about 0.03% of total T4 in plasma is unbound. The half-life of elimination of T4 is 6 to 7 days. In hyperthyroidism, the half-life is shortened to 3 or 4 days, whereas in hypothyroidism it may be 9 to 10 days. In conditions associated with reduced protein in plasma, such as nephrosis or hepatic cirrhosis, or when binding to protein is inhibited by certain drugs, the half-life of T4 may be shortened. The liver is the major site of degradation of Thyroid hormones. T4 is conjugated with Glucuronic and Sulphate conjugates through the Phenolic hydroxyl group and excreted in the urine. There is an enterohepatic circulation of the Thyroid hormones, since they are liberated by hydrolysis in the intestine and reabsorbed. Because of the long half-life of T4, a steady blood level of the biologically more active T3 can be obtained from a single daily dose of Levothyroxine. Therefore, variations in the therapeutic effect are unlikely once the correct dosage has been established.

Pregnancy & Breastfeeding

Pregnancy Category A. Levothyroxine requirements may increase during pregnancy. Although thyroid hormones are excreted only minimally in human milk, caution should be exercised when giving it to a nursing woman. However, adequate replacement doses of Levothyroxine are generally needed to maintain normal lactation.

⚠️ Talk to your doctor before taking any medicine during pregnancy or while breastfeeding.

Dosage Reference

⚠️ Reference ranges from official sources only. Consult your doctor for personalized dosing.

Adults: Adult dose: The initial starting dose is usually 25 to 50 mcg per day, with gradual increases at intervals of 6 to 8 weeks as needed. The Levothyroxine Sodium dose is generally adjusted in steps of 12.5 to 25 mcg until the patient with primary hypothyroidism is clinically euthyroid and the serum TSH level has returned to normal. In patients with severe hypothyroidism, the initial dose is 12.5 to 25 mcg per day, with increases of 25 mcg per day every 2 to 4 weeks, along with clinical and laboratory assessment, until the TSH level is normal. In patients with secondary (pituitary) or tertiary (hypothalamic) hypothyroidism, the Levothyroxine Sodium dose should be adjusted until the patient is clinically euthyroid and the serum free T4 level is restored to the upper half of the normal range. For patients older than 50 years, or patients under 50 years of age with underlying heart disease, the dose is 1.7 mcg per kg per day. Pediatric dosage for newborns: the recommended starting dose is 10 to 15 mcg per kg per day. A lower starting dose should be considered in infants at risk of heart failure, and the dose should be increased every 4 to 6 weeks as needed based on the clinical and laboratory response to treatment. In infants with very low serum T4 concentration (below 5 mcg per dL) or an undetectable level, the recommended initial starting dose is 50 mcg per day of Levothyroxine Sodium. Pediatric dosage for infants and children: in children with chronic or severe hypothyroidism, the initial dose is 25 mcg per day, with increases of 25 mcg every 2 to 4 weeks until the desired effect is achieved. Hyperactivity in an older child can be reduced if the starting dose is one fourth of the recommended full replacement dose, with the dose then increased weekly by an amount equal to one fourth of the full recommended replacement dose, until the full recommended replacement dose is reached. From 0 to 3 months, the dose is 10 to 15 mcg per kg per day. From 3 to 6 months, it is 8 to 10 mcg per kg per day. From 6 to 12 months, it is 6 to 8 mcg per kg per day. From 1 to 5 years, it is 5 to 6 mcg per kg per day. From 6 to 12 years, it is 4 to 5 mcg per kg per day. Over 12 years but with growth and puberty not yet complete, it is 2 to 3 mcg per kg per day. Once growth and puberty are complete, it is 1.7 mcg per kg per day. The dose should be adjusted based on the patient's clinical response and laboratory parameters.

Side Effects & Interactions

Side Effects

No side-effect data recorded yet.